DC MOTOR QUALITATIVE QUESTIONS a What is armature reaction H

DC MOTOR QUALITATIVE QUESTIONS

(a) What is armature reaction? How does it affect the operation of a DC machine?

(b) What types of losses are present in a DC machine?

(c) How can the speed of a shunt DC motor be controlled? Explain in detail.

(d) What are the principal characteristics of a series DC motor? What are its uses?

(e) What are the characteristics of a cumulatively compounded DC motor?

(f) What are the problems associated with a differentially compounded DC motor?

(g) Why is a starting resistor used in DC motor circuits?

Solution

(a) Armature Reaction :- The effect of magnetic field set up by the armature current on the distribution of flux under the main poles of a dc machine is known as armature reaction.

Affects of armature reaction:- 1. It reduces main flux which in turns reduced induced emf. 2. Commutation problem, 3. increase iron losses.

(b) Losses in DC mainly divided into two catagories 1. Constant losses , 2. Variable losses

1. Constant losses : a) Iron losses, friction and windage losses.

2. Variable losses : a) armature copper losses, b) field copper losses.

(c) Speed of DC shunt motor can be controlled using two methos :-

1. Armature voltage control method :- In this method the voltage applied to armature is controlled using rectifier circuits or potential dividers. This method of filed control is used when under base speed control is required.

2. Filed current control method : In this method current passed through filed is decreased using resistors in series with field. This method of spped control is used when above rated speed control is required.

(d) Principle characteristics of a series DC motor these are used as variable speed motors. DC motors develops large starting torque which makes these motors to use in application high starting torque is required. In DC series motor field flux is proportional to armature current and hence with increase in load current the speed decreases.

e) Cumulative compound DC motors have the characterstics which combines shunt and series motors. As load increases the flux due series filed increases and causes the torque increase . It has high starting torque characterstics as series motor. These are used to handle sudden heavy loads applied to drive.

f) Problems associated with a differential compound dc motor are field may be weakened too much and motor may tend to race, taking very large current. This results in series filed winding overpowering the shunt causing the motor reverse.

g) Starting resistance is used in DC motor circuit to decrese starting inrush current. This is because the armature of DC motor has very small resistance, during starting no back emf was developed to oppose the applied voltage. So it leads to very high starting current. To prevent motors from burning we need to keep starting resistance to prevent high inrush currents.